The electroluminescence of organic materials has become a field of intensive research since first being discovered in 1953 (Bernanose et al., J. Chim. Phys. 1953, 50, 65). The known advantages of organic materials for producing light, such as for example low reabsorption, high quantum yields and also the possibility of adapting the emission spectrum by varying the molecular structure in a relatively simple manner, could be exploited in recent years through constant development in materials research and the implementation of new concepts for effectively injecting and transporting the charge carriers into the active emission layer of an organic light-emitting element. The first display devices based on such so-called organic light-emitting diodes have already found their way onto the market, and organic light-emitting diodes will in future be firmly established as a concept alongside liquid crystal displays and displays formed of inorganic light-emitting diodes. Another market which is open to organic light-emitting diodes due to their special property of being able to emit light over a large surface area and homogeneously into the half-space is the lighting field.
The step towards mass production increases the pressure to minimize individual cost factors of the product, costs of the raw material, or even steps in the production process. One of the main cost factors is the transparent electrode material indium tin oxide (ITO) currently used, both on account of the high demand for indium, which rarely occurs naturally on Earth and is therefore expensive, and also due to the cost-intensive process for applying ITO to the substrate by sputtering. If a transparent light-emitting diode is to be produced, i.e. if ITO is therefore also to be applied as counter-electrode to the organic layers, the production process becomes even more difficult since complicated measures are necessary in order to protect the organic layers against damage caused by the high-energy particles that occur during the sputtering process.
As an example, a highly conductive polymer layer for alternative use as an electrode is proposed in the document DE 103 35 727 A1. Such polymer layers applied in the liquid form achieve conductivities of up to 550 S/cm. This obviously solves the problem of the expensive starting material, since indium is not used. However, one disadvantage of this invention is that an additional step is still required in order to apply the polymer electrode to the substrate. Particularly for producing organic light-emitting diodes which consist of a sequence of amorphous thin layers applied by vapor deposition in vacuo, the application of the polymer electrode by spin-coating at normal pressure under simultaneously dust-free conditions complicates the production process and therefore makes it more expensive. Furthermore, it appears to be impossible to use such a method to produce transparent light-emitting diodes since, during the application of the polymer layer by spin-coating, solvents are used which generally also dissolve the underlying organic thin layers and thus unpredictably alter or even destroy the device.
One further development of transparent organic light-emitting diodes consists of stacked transparent light-emitting diodes (see for example Gu et al., J. Appl. Phys. 1999, 86, 4067). In said document, a number of transparent light-emitting diodes are applied sequentially to a substrate in a stacked manner, wherein in each case two successive light-emitting diodes have a transparent electrode in common. In order to be able to actuate the light-emitting diodes of the stack individually, transparent electrodes are guided out of the sides and contacted. To this end, a high lateral conductivity of the electrodes is required, and for this reason usually ITO is used. This leads to the same problems as mentioned above for transparent light-emitting diodes.
However, the literature to date has not yet disclosed organic layers which can be applied by vapor deposition and which have conductivities comparable to the materials applied from the liquid phase. Organic semiconductors applied by vapor deposition in general have a very low conductivity, particularly in their amorphous phase, so that, despite the effective increase in conductivity by several orders of magnitude due to doping with suitable dopants, the lateral movement of the charge carriers in a layer under vertical bias with partially non-overlapping electrodes is negligible. See in this regard FIG. 1, which shows a schematic transverse view of the described arrangement. Provided here are an electrically non-conductive substrate 1, electrodes 2, 4 which at least partially do not overlap one another, and one or more organic layers 3 in the electric field of the two electrodes 2 and 4.
For such arrangements, the conductivities reported to date for organic layers of high transparency which are applied by vapor deposition are not high enough to achieve sufficient lateral transport. Although the high intrinsic electron mobility of C60 when doped with suitable donor-type molecules (see for example Werner et al., Adv. Func. Mater. 2004, 14, 255) may lead to high effective mobilities and conductivities (sometimes in the region of σ=0.1 S/cm), these are far from sufficient for conductive grids in the case of a resolution of approximately 100 μm which can be set for example by means of printing techniques.
Important factors for the efficiency of electroluminescent light-emitting devices are, in addition to the yield when converting electrical energy in the emission layer into light, the injection of electrons from the cathode and of holes from the anode into the respectively adjoining layer and also the capability of the individual layers for charge carrier transport. It has been found that an organic material generally does not intrinsically possess all the necessary properties, so that sometimes different materials have to be used for different functions in order to obtain an efficiently functioning component.
As hole injection and hole transport materials (Hole Transport Material—HTM), materials with work functions IP>4.5 eV and hole mobilities μh>1×10−5 cm2/Vs are generally used, in order to allow good injection from the anode (ITO) and efficient transport of the holes. Examples of HTMs are phthalocyanines such as CuPc, so-called starburst molecules such as MTDATA or else benzidines such as TPD and NPD (see for example Adachi et al. (2003), “Design Concept of Molecular Materials for High-Performance OLED”, in: Shinar (ed.), Organic Light-Emitting Devices, Springer, New York, page 43).
By contrast, the materials used as electron injection and electron transport materials (Electron Transport Material—ETM) typically have electron mobilities μe>1×10−6 cm2/Vs and electron affinities EA<3.5 eV. Here, the suitable choice of EA is essentially determined by the EA of the emitter material used. Typical examples are oxadiazoles, triazoles, quinolines or thiophenes (see for example Hughes et al., J. Mater. Chem. 2005, 15, 94).
The doping of hole transport materials with acceptor-type dopants and of electron transport materials with donor-type dopants has proven to be advantageous (Pfeiffer et al., Org. Electron. 2003, 4, 89). Due to the increase in the concentration of free charge carriers in the layer which is achieved as a result, both the conductivity and the effective charge carrier mobility are improved. If a doped organic layer is used in a Schottky contact, the depletion zone becomes much thinner than in the undoped case due to the likewise increased space charge density. The injection of the charge carriers from an electrode into the transport layer can thus be significantly improved by doping the transport layer. Last but not least, this leads to a greater independence from the work function in the choice of electrode material, so that a larger selection of materials can be used as the electrode material.
Fullerenes, in particular buckminsterfullerene C60, have also been the subject of intensive research since their discovery in 1985 and are used for example as an acceptor material in organic solar cells (see for example U.S. Pat. No. 6,580,027 B2).
A method for producing C60 and C70 in relatively large quantities was developed (see WO 92/04279). Since then, production methods for fullerenes have formed the subject of continuous further development, so that nowadays fullerenes are available as a very inexpensive starting material.
To date, only a few attempts have been made to use fullerenes in organic light-emitting diodes. It has been found that fullerenes in organic light-emitting diodes are not readily useful and in many cases even leads to a worsening of the properties.
For example, it has been shown (Day et al., Thin Solid Films 2002, 410, 159) that even submonolayers of C60 between the ITO anode and the hole transport layer make injection more difficult and lead to a worsening of the component characteristics. On the other hand, in components which conduct only holes, an increase in current density at the same voltage has been found (Hong et al., Appl. Phys. Lett. 2005, 87, 063502) when a thin C60 layer is added between the anode and the organic layer, with the authors attributing this to a surface dipole and not to a layer property of the fullerene.
Yuan et al. (Appl. Phys. Lett. 2005, 86, 143509) have found that holes can be better injected from the ITO anode into an NPB layer doped with five percent by weight of C60 than into a pure CuPc layer. However, the injection is even better if an undoped NPB layer is used. Lee et al. (Appl. Phys. Lett. 2005, 86, 063514) show that C60 in TDAPB acts as a weak electron acceptor, and the effective hole mobility is increased. In addition, since free electrons from leakage currents in an OLED comprising C60:TDAPB as the hole transport layer are effectively captured by C60 and are thus unable to destabilize the TDAPB, the service life increases for the same initial brightness.
Despite the high electron mobility μe˜8×10−2 cm2/Vs, the use of C60 as an electron transport layer is opposed firstly by the relatively high electron affinity of ˜4 eV. Lu et al. (US 2004-214041 A1) nevertheless use C60 in conjunction with a LiF/Al cathode as an electron transport layer, with the LiF injection layer being absolutely necessary in this case. Yasuhiko et al. (PCT WO 2005-006817 A1) use separately produced Li-containing C60 as the electron transport layer.